This invention relates to the gas pressurization of liquid carbon dioxide (CO.sub.2) cylinders used, for example, by bars and soda fountains to dispense carbonated drinks.
The cylinders used in such cases are basically two metal containers, one within the other. The space between the containers acts as a thermal barrier because of insulation and a vacuum. Insulation prevents radiant heat from entering the inner container and the vacuum prevents heat convection or conduction from reaching the inner container.
As CO.sub.2 gas is withdrawn from the liquid CO.sub.2 storage cylinder, the gas pressure in the cylinder drops causing some liquid to turn to gas to restore the pressure in the cylinder. The gasification of liquid CO.sub.2 is endothermic and, therefore, the temperature of the liquid contents of the cylinder drops. If gasification were permitted to go on continuously without heat input, solid CO.sub.2 would form in the cylinder and the flow of CO.sub.2 gas out of the cylinder would be minimal.
A widely used liquid CO.sub.2 storage cylinder is sold under the trademark Liquidator by Taylor-Wharton Company of Harrisburg, PA. The Liquidator which is essentially representative of competitive liquid CO.sub.2 storage cylinders is provided with a tube that permits the flow of liquid CO.sub.2 from the bottom of the inner container and completely out of the cylinder so that the liquid can be warmed and gasified by ambient air. The resulting CO.sub.2 gas is returned to the top of the cylinder to maintain the desired high pressure in the vapor space above the liquid CO.sub.2 in the cylinder while CO.sub.2 gas is being withdrawn for preparing carbonated beverages or other use.
The desired pressure in the cylinder is generally maintained at about 250 pounds per square inch gauge (psig). A relief valve connected to the cylinder will automatically vent CO.sub.2 gas if the pressure builds up beyond the set limit and when the pressure drops a predetermined amount, liquid CO.sub.2 will automatically flow out of the cylinder through the tube previously described for maintaining the desired pressure in the cylinder. During periods of large withdrawal of CO.sub.2 gas for its intended use, the rapid drop in pressure may not be restored quickly enough by the flow of liquid CO.sub.2 through the tube particularly if the tube is exposed to low ambient temperatures; in such case, the pressure of the CO.sub.2 gas withdrawn from the cylinder may be too low for its intended use.
This serious limitation of the tube for pressure restoration is exacerbated by contaminants such as moisture and hydrocarbons (oil) in liquid CO.sub.2 supplied by producers. Inasmuch as the contaminants are much less volatile than CO.sub.2, their content in the cylinder keeps increasing with each new delivery of liquid CO.sub.2 from the producer. Therefore, when the temperature of the liquid CO.sub.2 in the cylinder drops appreciably because of a rapid pressure drop during a period of high CO.sub.2 gas utilization, the contaminants will congeal and plug the tube connected to the bottom of the cylinder. Thereupon, the tube loses its pressure restoration function and the cylinder cannot dispense CO.sub.2 gas at the pressure required by the user.
Accordingly, a principal object of this invention is to provide low-pressure (usually not over 300 psig), insulated cylinders for liquid CO.sub.2 storage with reliable means for restoring pressure therein when CO.sub.2 gas is withdrawn rapidly.
Another important object is to provide such reliable pressure restoration means that can be simply installed in such existing CO.sub.2 cylinders.
Still another important object is to provide improved means for measuring the quantity of liquid CO.sub.2 in storage cylinders.
Other features and advantages of the invention will be apparent from the description which follows.